The present invention relates to an OFDM (Orthogonal Frequency Division Multiplex) modulation method and an OFDM modulator used in digital transmission apparatuses of OFDM modulation system.
In recent years, digitization of television broadcasting have been studied. As a modulation system for television broadcasting, the adoption of the OFDM modulation system is regarded as promising.
The OFDM modulation system is one kind of multi-carrier modulation system. In the OFDM modulation system, a large number of digital modulated waves are added together. As the modulation system of each carrier at this time, the QPSK (quadrature phase shift keying) system or the like is used. According to this system, a composite wave (OFDM signal) as shown in FIG. 1 can be obtained.
FIG. 1 illustrates the case where the number of carriers is 24. In FIG. 1, T.sub.S denotes an effective symbol duration (duration of all effective symbols) of one symbol data.
Expression of this OFDM signal using a numerical expression will now be described.
Representing the QPSK signal of each carrier by .alpha..sub.k (t), it can be expressed by equation (1).
.alpha..sub.k (t)=a.sub.k (t).times.cos (2.pi.kft)+b.sub.k (t).times.sin (2.pi.kft) (1)
Here, k represents a carrier number, and a.sub.k (t) and b.sub.k (t) are data of a kth carrier, and assume the value of [-1] or [1].
Assuming the number of carriers is N, the OFDM signal is a combination of N carriers. Denoting this by .beta..sub.k (t), it can be represented by equation (2). ##EQU1##
Assuming now that each component has coefficient values a.sub.k =0 and b.sub.k =1 in the equation (1) and N=24 in the equation (2), waveforms of the coefficient values .alpha..sub.k (t) in the equation (2) are exemplified in FIG. 1. Representing the equation (2) in the form of a frequency spectrum, a spectrum shown in FIG. 2 is obtained.
In the OFDM system, a guard interval is typically added to each effective symbol duration of the composite carrier signal in order to mitigate the influence of the multi-path. In other words, a guard interval .DELTA.T is added to an effective symbol duration V.sub.S as shown in FIGS. 3 and 4.
FIG. 3 illustrates an example of the case where a guard interval has been added to a carrier wave of k=1 for simplifying the description. FIG. 4 illustrates an example of the case where a guard interval has been added to a composite carrier signal obtained by combining N=448 carriers.
As shown in FIG. 3, a waveform (a) of an interval .DELTA.T/2 located in a start portion of the effective symbol duration V.sub.S is added after an end edge of the effective symbol duration as a rear guard interval waveform (a') of an interval .DELTA.T/2. In the same way, a waveform (b) of an interval .DELTA.T/2 located in an edge portion of the effective symbol duration V.sub.S is added before a start edge of the effective symbol duration as a front guard interval waveform (b') of an interval .DELTA.T/2. By the sum total of these front and rear guard interval waveforms (b') and (a'), a guard interval of an interval .DELTA.T is added to one effective symbol duration V.sub.S. The entire symbol duration containing the effective symbol duration V.sub.S and the guard interval .DELTA.T, i.e., the entire symbol duration (symbol duration with guard) becomes T.sub.S.
By the way, (c) is a rear guard interval added to an immediately preceding effective symbol duration, and (d) is a front guard interval added to an immediately succeeding effective symbol duration. At connection points (changeover points) t.sub.1 and t.sub.2 of the entire symbol duration, the waveform becomes discontinuous. Especially at the time t.sub.1 forming the connection point between the entire symbol duration of X=0 and the entire symbol duration of X=1, the waveform step (level change) becomes large and side lobes are generated as described later.
In the above described example, the front and rear guard intervals having the same interval .DELTA.T/2 are added to the start edge and the end edge of the effective symbol duration, respectively. Alternatively, a front guard interval and a rear guard interval differing in duration and having total duration equivalent to .DELTA.T may be added to the start edge and the end edge of the effective symbol duration, respectively.
Alternatively, a guard interval having duration of .DELTA.T may be added to the start edge or the end edge of the effective symbol duration. FIG. 5 is a waveform diagram of the case where a guard interval having duration of .DELTA.T has added to the end edge of the effective symbol duration for the carrier wave of k=1.
FIGS. 6A and 6B are basic configuration block diagrams of an OFDM modulation/demodulation apparatus using a conventional technique shown in, for example, JP-A-7-321762. Hereafter, the modulation/demodulation operation of the OFDM signal will be described by referring to FIGS. 6A and 6B. As illustrated, an IFFT (Inverse Fast Fourier Transform) unit 81 is used for the OFDM modulation, and a FFT (Fast Fourier Transform) unit 95 is used for demodulation.
In a system of adding guard intervals to a temporal waveform generated by modulating an input data train into a large number of (sub)carriers, a modulation unit (sending side)(OFDM modulator) for conducting orthogonal frequency division multiplex (OFDM) includes an IFFT unit (81) for conducting inverse fast Fourier transform (IFFT) processing to modulate the input data train into a large number of (sub)carriers, a guard interval adder for adding a guard interval to each effective symbol duration of a combined carrier signal supplied from the IFFT unit (81) and outputting a resultant signal, and an quadrature processor (84) for applying quadrature modulation to the signal supplied from the guard interval adder and outputting an OFDM signal.
In other words, in the sending side (modulation unit) T shown in FIG. 6A, inverse Fourier transform is conducted in the IFFT unit 81 by regarding an in phase component I of each carrier as real part data R.sub.f and a quadrature component Q as imaginary part data I.sub.f. Thus, a real part signal R and an imaginary part signal I in the time domain are obtained.
To this signal, a signal corresponding to the guard interval is added in the guard interval adder 82. Resultant signals R.sub.g and I.sub.g are subject to D/A conversion in a digital-to-analog (D/A) converter 83 (83a and 83b). By using a carrier signal having a frequency f.sub.C supplied from an oscillator 85 for an analog signal of the real part signal R.sub.g and using a carrier signal shifted in phase by 90.degree. by a phase shift circuit 86 for an analog signal of the imaginary part signal I.sub.g, quadrature modulation is conducted in the quadrature modulator 84. Resultant signals are combined in a synthesizer (adder) 87. An OFDM signal is thus obtained. In FIG. 6A, 84a and 84b denote multipliers.
In the receiving side (demodulation unit) R shown in FIG. 6B, operation opposite to that of the sending side is conducted. An output obtained by demodulating the received OFDM signal in an quadrature demodulator 91 with a carrier signal of a frequency f.sub.C supplied from an oscillator 93 is taken out as a real component. An output obtained by demodulating the received OFDM signal in the quadrature demodulator 91 with the carrier signal shifted in phase by 90.degree. in a phase shifting circuit 92 is taken out as an imaginary component. These demodulated signals are converted to digital signals by an analog-to-digital (A/D) converter 94 (94a, 94b). The digital signals are subject to Fourier transform in a FFT unit 95. OFDM demodulated signals are thus obtained. In FIG. 6B, 91a and 91b denote multipliers.
When conducting fast Fourier transform, a timing signal regenerator 96 is used to attain timing as shown in the receiving side R of FIG. 6B.
For attaining the timing of this fast Fourier transform, signals for timing extraction (hereafter referred to as synchronizing signals) are embedded at the rate of m symbols every M symbols (where m&lt;M) on the sending side T as shown in FIG. 7.
Here, one symbol means the entire symbol duration T.sub.S obtained by adding the guard interval .DELTA.T to the effective symbol duration V.sub.S.
By extracting the synchronizing symbols on the receiving side R, symbol boundaries (connection points) are calculated and timing of fast Fourier transform is attained.
A timing signal regenerator used for this purpose can be implemented by using, for example, a circuit having a configuration as shown in FIG. 8.
To be concrete, a pattern of the synchronizing symbol (such as, for example, a sine wave of a single frequency) 21a is stored in a memory 22 beforehand by using a timing signal regenerator. An A/D converted OFDM signal 21b and the pattern 21a stored in the memory 22 are input to a correlator 21. At the time when the pattern 21a supplied from the memory 22 coincides with the pattern of the OFDM signal 21b, a pulse 96a functioning as a timing signal is generated and supplied to the FFT unit 95. This pulse 96a is used in the FFT unit 95 as the start timing of the fast Fourier transform.
In FIG. 2, a waveform of one out of a plurality of carrier signals (i.e., a waveform of a carrier frequency f.sub.C.times.m) is indicated by a thick line above the OFDM spectrum. From the waveform, it is understood that a main spectrum occurs at a carrier frequency f.sub.C.times.m in an OFDM signal and spectra having frequencies such as f.sub.C.times.(m-1) and f.sub.C.times.(m+1) occur at fixed frequency intervals adjoining to the frequency f.sub.C.times.m while repetitively becoming smaller.
In the OFDM signal having such a characteristic and including a combination of a plurality of carriers, carriers are in mutual orthogonal relations. Therefore, the OFDM signal has such a property that side lobes caused by the discontinuity between symbols of carriers become the 0 level in main spectra of other carriers and the carriers do not affect each other. On the other hand, unnecessary leakage power appear outside the band.
Assuming that the OFDM signal shown in FIG. 2 has a carrier spacing frequency of f.sub.C and includes N carriers, the spectrum of such a sent OFDM wave will now be qualitatively described. In this case, the required bandwidth of the OFDM signal becomes f.sub.C.times.N.
First of all, in the required bandwidth f.sub.C.times.N, main spectrum components of a total of N carriers occur at intervals of the frequency f.sub.C so as to form a rectangular shape.
On the other hand, outside the required bandwidth f.sub.C.times.N, side lobes of N carriers occur overlapping each other while being attenuated. As a result, unnecessary leakage spectra occur outside the required bandwidth f.sub.C.times.N in a slope form as illustrated.
As for the level of the leakage spectra appearing outside the band, a level of about -20 dB with respect to each of the carriers contained in the required band is maintained in the vicinity of the required band. Even at a frequency located far away from the required band, the level eventually falls only to approximately -40 dB.
In typical transmission systems, all of the frequency bandwidth (such as, for example, 9 MHz) allowed to be used is used in order to secure the greatest possible transmission capacity.
On the other hand, frequency regions adjacent to the required band are utilized for other communication services. If there is a leakage spectrum other communication is disturbed.
As for the leakage spectra outside the required band, only leakage spectra having a predetermined level or below (such as, for example, -40 dB or below) are allowed.
In the conventional technique the leakage spectra outside the required band are suppressed by using a filter.
Unnecessary radiation in a frequency region located far away from the required band can be easily removed by only inserting a BPF (bandpass filter) having a gentle characteristic in a high frequency portion, an intermediate frequency portion, or a baseband portion.
The above described conventional technique has problems caused by use of a filter that are not taken into consideration. Further, by use of a filter it is difficult to prevent the cost and size of the apparatus from increasing.
For removing the radiation having an in-band carrier ratio of about -20 dB appearing in the vicinity of the required frequency band by using a filter as in the conventional technique, there is needed a filter attenuating the output level by at least 20 dB in the vicinity of the required band located outside the required band.
For obtaining sharp attenuation as large as 20 dB in such a small frequency difference, however, a high performance filter having a large number of stages, i.e., a large sized, expensive filter is needed. This makes it difficult to reduce the size and cost of the apparatus.
On the other hand, if a filter satisfying practical conditions concerning the cost and so on is used, a sharp attenuation characteristic cannot be expected. In the case of the OFDM signal having a sharp spectrum characteristic, therefore, even the carriers in the required band are attenuated.
In this case, therefore, it is difficult to avoid the influence upon other communication with the exception of adopting such a design as to decrease the required bandwidth and consequently the transmission capacity.